1. Field of the Invention
This invention relates generally to the field of iron founding and steelmaking and more particularly to a process for melting iron to produce a carbon-containing hot metal from scrap iron, scrap steel, pig iron, direct reduced iron or mixtures of these ferrous materials without the use of substantial amounts of coke.
2. Description of the Prior Art
The iron founding industry is well established in the United States and throughout the world. It includes, in broad categories, the large integrated steel mills which produce a wide variety of finished steel products, the so-called mini-mills which produce a more limited range of steel products, and iron and steel foundries which produce iron and steel products from scrap iron, scrap steel or pig iron.
For the large integrated steel mills, the basic melting unit is usually the blast furnace which produces hot metal containing around 4.0% carbon together with impurities including silicon, sulfur, phosphorous and manganese from raw materials comprising iron ore, metallurgical coke and various fluxes. Smaller steelmaking plants and foundries frequently employ a cupola or an electric furnace as the basic melting unit.
The cupola is a shaft-type furnace wherein the burden comprises a ferrous material such as scrap iron, scrap steel or pig iron together with metallurgical coke and fluxes. The energy, in the form of heat, required to melt the ferrous burden is produced by combustion of the coke with air which may be preheated or enriched with oxygen. In recent years, there has been a shortage of metallurgical grade coke and its cost has increased from about $70/ton to about $175/ton. As coke comprises about 12% of the cupola charge, its cost is a matter of some concern. Additionally, the coke is a source of gaseous and particulate emissions. The energy cost to produce molten iron using a cupola is now estimated to be about $21/ton.
As a result of the raw material and pollution problems related to the cupola, the electric furnace has become more and more popular as a basic melting unit. The electric furnace may be of either the induction type or the electric arc type. In either case, the thermal energy to perform the melting function is provided by electricity. Coke, except as a possible source of carbon, is not required. Although the electric furnace avoids the problems related to the use of coke, the capital costs of the electric furnace and its associated electrical distribution system are comparable to those of the hot blast cupola. Additionally, the energy cost to melt a ton of iron in the electric furnace is estimated to be about $45, or a figure substantially in excess of that for the cupola.
The disadvantages of the cupola and electric furnace melters have engendered interest in alternative methods for melting ferrous burdens. One such alternative is the use of natural gas, fuel oil or powdered coal to fire a cupola or shaft-type furnace. Such an approach potentially avoids certain of the problems associated with the use of coke as well as the cost of electrical energy as natural gas is one of the cleanest and least expensive fuels available and its supply, at least for the foreseeable future, appears to be adequate. The use of fuel oil or pulverized coal as a source of energy is somewhat less favorable since its cost, availability and combustion equipment complexity are more difficult to predict.
The use of fluid fuel as a source of energy for a ferrous melter has been employed in the so-called "cokeless cupola" development in England, as described in Taft et al. U.S. Pat. No. 3,802,678. A process for reducing ore and then melting in a gas-fired or oil fired shaft furnace designed to operate with a reducing atmosphere is disclosed in Syska U.S. Pat. No. 4,111,687. Syska discloses the combustion of a rich fuel/oxidant mixture to produce a reducing atmosphere within the melting unit and the use of this atmosphere to preheat and reduce the charge of iron ore and to melt the reduced charge.
The present invention represents a further development of certain of the concepts set forth in U.S. Pat. No. 4,111,687.